Actuatable occupant restraint systems, such as air bags, for vehicles are well known in the art. Such restraint systems include one or more collision sensing devices for sensing vehicle crash acceleration (vehicle deceleration). Air bag restraint systems further include an electrically actuatable igniter, referred to as a squib. When the collision sensing device senses a deployment crash event, an electrical current of sufficient magnitude and duration is passed through the squib to ignite the squib. When ignited, the squib initiates the flow of inflation fluid into an air bag from a source of inflation fluid, as is known in the art.
Certain known collision sensing devices used in actuatable occupant restraint systems are mechanical in nature. Still other known actuatable occupant restraint systems for vehicles include an electrical transducer, such as an accelerometer, for sensing vehicle crash acceleration. Systems using an accelerometer as a crash or collision sensor further include some circuitry, e.g., a controller, for monitoring the output of the accelerometer. The accelerometer provides an electrical signal having an electrical characteristic indicative of the vehicle's crash acceleration. The accelerometer is operatively connected to a controller, such as a microcomputer, which performs a crash algorithm on the acceleration signal for the purpose of discriminating between a deployment and a non-deployment crash event. When a deployment crash event is determined to be occurring, the restraint is actuated, e.g., an air bag is deployed.
One particular type of occupant restraint system known in the art is a multi-stage occupant restraint system that includes more than one actuatable stage associated with a single air bag. In a multi-stage air bag restraint system, air bag inflation is the result of the control of a multi-stage inflator. Such multi-stage air bag systems typically have two or more separate sources of inflation fluid controlled by actuation of associated squibs. Known control arrangements control the actuation of the multiple stages based on a timer function. A problem arises in monitoring for a beginning of the crash event to start the timer. False starts (and endings) could occur due to signals resulting from road noise.
U.S. Pat. No. 3,966,224 is directed to a multi-stage air bag restraint system having two squibs. Under certain types of crash conditions, a first stage is actuated followed by actuation of a second stage a predetermined time after actuation of the first stage. If the crash acceleration is greater than a predetermined level, both stages are simultaneously actuated.
U.S. Pat. No. 4,021,057 is directed to a multi-stage air bag restraint system having a plurality of firing elements for gas generators. Crash velocity is compared against a plurality of threshold values for control of the plurality of squibs and, in turn, control of the inflation rate of the air bag.
U.S. Pat. No. 5,400,487 is directed to an air bag restraint system having a plurality of separately controlled gas generators actuated at selected times in a selected order to control the air bag's inflation profile. The selective triggering is a function of both the crash type extrapolated from past received acceleration data and the occupant position based on received occupant position data.
U.S. Pat. No. 5,411,289 is directed to an air bag restraint system having a multiple level gas generation source. "The electronic control unit is responsive to a combination of sensed inputs from the temperature sensor, the seat belt sensor, and the acceleration sensor for determining both an optimum gas generation level and inflation sequence times for controlling the multiple level gas generation source." (Abstract of '289 patent)
Many types of crash algorithms for discriminating between deployment and non-deployment crash events are known in the art. Algorithms typically are adapted to detect particular types of crash events for particular vehicle platforms. One example of such an algorithm is taught in U.S. Pat. No. 5,587,906 to McIver et al. and assigned to TRW Inc.
Air bag restraint systems are also known to require more than one sensor for detection of a deployment crash event. Often, the plural sensors are arranged in a voting scheme in which all the sensors must "agree" that a deployment crash event is occurring before restraint actuation is initiated. In certain known arrangements having a first and second sensor, the second sensor is referred to as a "safing sensor." Air bag actuation occurs only if the first sensor and the safing sensor indicate a deployment crash event is occurring.